Microwave applicator system with cylindrical resonant cavity

ABSTRACT

A cylindrical microwave resonant cavity is excited in the TM010 mode to generate an electric field extending parallel to the axis of the cavity and having a maximum intensity along the axis. A filament or thread is conveyed axially through the cavity and heated. The cavity housing is formed into two sections along radial planes. The sections are hinged about a line parallel to the axis to facilitate opening the housing and cleaning its interior. Liners may be provided to cover the interior of each section. A slot is formed between the sections opposite the hinge line for threading the filament into its operative axial position. Clamps are provided to resiliently urge the sections together, yet the clamps permit adjustment of the angle of inclination between the sections by means of a screw mechanism to tune the resonant frequency of the cavity to the operating frequency of the microwave source. Air, preheated by cooling the source, is routed through the feed guide into the cavity to purge vapors from the cavity and prevent accumulation of residue. An electrical interlock is provided to inhibit operation of the microwave source when the sections are separated beyond a predetermined limit.

United States Patent Johnson June 27, 1972 [54] MICROWAVE APPLICATORSYSTEM Primary Examiner-R. F. Staubly WITH CYLINDRICAL RESONANTAssistant Examiner-Hugh D. Jaeger CAVITY Attorney-Carl C. Batz 57ABSTRACT [72] Inventor: Ray M. Johnson, Danville, Calif. 1

A cylindrical microwave resonant cavity is excited in the Asslgnee:cryodl'y corpon'ilmi San Ramon: Callf- TM mode to generate an electricfield extending parallel to 22 Fl (1: A l 3 1970 the axis of the cavityand having a maximum intensity along 1 l e the axis. A filament orthread is conveyed axially through the [21] Appl. No.: 25,402 cavity andheated. The cavity housing is formed into two sections along radialplanes. The sections are hinged about a line parallel to the axis tofacilitate opening the housing and clean- I52] U.b. 9| ..2l9/l0.55 ingits Marion Liners may be provided m cove, the interior of I "Hosb 9/06each section. A slot is formed between the sections opposite [58] Fieldof Search l 9/l0.55 the hinge line for threading the filament into itsoperative axial position. Clamps are provided to resiliently urge thesections [56] Rekremes Cited together, yet the clamps permit adjustmentof the angle of inclination between the sections by means of a screwUNITED STATES PATENTS mechanism to tune the resonant frequency of thecavity to the operating frequency of the microwave source. Air,preheated 3,339,054 8/1967 Deaton ..2l9/l0.55 by cooling the source, isrouted through the feed guide into 3,407,279 10/1963 Greenberg thecavity to purge vapors from the cavity and prevent accu- 3,457,3857/1969 Cumming ....2l9/10.6-1 mulation of residue. An electricalinterlock is provided to in- 3,461,26l 8/1969 Lewis et al........219/l0.6l hibit operation of the microwave source when the sectionsare 3,480,753 I l/ I969 Wilson et al. ..2l 9/ 10.55 separated beyond apredetermined limit.

17 Claims, 8 Drawing Figures i J J l l I l [Ilium 1 0 TERMINATION 7STAB! LIZ/N6 NE T WORK I I IL J0 F'A'TENTEDJUNZ'I m2 SHEET 10F 2STAB/L/ZING NETWORK TERMINATION STABILIZING NETWORK PATENTEDJum m2 SHEET2 OF 2 MICROWAVE APPLICATOR SYSTEM WITH CYLINDRICAL RESONANT CAVITYBACKGROUND AND SUMMARY The present invention relates to microwaveheating applicators; and more particularly, it relates to a system forapplying microwave energy to a lossy dielectric object passing through aresonant cylindrical cavity.

Systems have been developed for applying microwave energy to objects forheating them. The application of microwave energy has been particularlyuseful in cooking food products. Among such developments have been batchtype ovens in which the food material is placed in a multimode cavity,continuous type microwave ovens in which the food is passed through amultimode cavity, and serpentine configurations of waveguides ofrectangular cross section in which the product (such as wallboard) ispassed through aligned slots in opposing broadwalls of a number ofwaveguide sections folded so that all of the slots are aligned.

A copending, co-owned application of mine entitled Continuous MicrowaveHeating Or Cooking System, Ser. No. 816,722, filed Apr. 16, 1969,describes a rectangular waveguide applicator excited in the TE modewherein the material being treated is conveyed through the applicatoralong the direction of power flow. The symbol TE refers to thetransverse electric field vector; and TM" refers to the transversemagnetic field vector.

Another copending, co-owned application of mine entitled Resonant CavityMicrowave Applicator, Ser. No. 852,374, filed Aug. 22, 1969 discloses asystem for heating a singleended filament including a resonant cavityhaving a cylindrical side wall and excited in the TM mode by a source ofmicrowave energy. The filament is conveyed axially through the cavityand inthe location of maximum field intensity. The present inventionrelates to improvements in this latter type of microwave heating system.

The applicator of the present invention is a cylindrical resonant cavitywhich is excited in the TM mode preferably via a hollow waveguide togenerate an electric field extending parallel to the axis of the cavityand having a maximum intensity along the axis. The cavity housing isformed into two sections divided by radial planes. The sections may, ofcourse, be half-sections; and they are hinged about a line parallel tothe axis of the cavity of facilitate opening and cleaning the interiorof the cavity. Removable liners of low dielectric loss factor may beprovided to cover the interior of each section. Thus, for example, whenthe applicator is being used to dry a filament that is impregnated withlatex, the movement of the filament as it is transported through theapplicator splashes the latex against the walls of the cavity. With thepresent invention, the down (i.e. inoperative) time of the applicator isgreatly minimized because the liners may simply be replaced aftershutting the microwave power off and opening the cavity about itshinges. After they are removed, the liners may be cleaned and re-used.Even if liners are not used, the ability to open the cavity about ahinge line greatly facilitates access to the interior of the cavity forcleaning.

An electric interlock is provided for preventing coupling of microwavepower to the cavity when the sections of the cavity housing areseparated beyond a predetermined limit.

At the input and exit apertures in the transverse plates of the cavitythere are formed outwardly extending semi-cylindrical neck members; andspring clamps are fitted about associated pairs of these neck membersfor holding the cavity sections resiliently together. A screw mechanismis provided external of the cavity for adjusting the separation of thehalfsections against the resilient urging of the spring clamps; and thisprovides a convenient mechanism for fine tuning of the resonantfrequency of the cavity. The slot formed between half sections andcommunicating with the axis of the cavity is enlarged along thehalf-sections of the cavity opposite the hinge to facilitate threadingof the filament into the cavity.

A probe is provided in one of the transverse end plates of the cavity ata peripheral location for monitoring the power coupled to the cavity toassist in matching the resonant frequency of the cavity to that of thesource, which may be a conventional magnetron oscillator.

The air in the cavity is purged by a source of air which is derived fromthe cooling air for the magnetron; and this aid is fed through the feedwaveguide to the cavity and exits through the apertures formed in thetransverse end plates of the cavity as well as the threading slot; Thus,the air fed to purge the cavity is preheated and by forcing it throughthe feed waveguide, the accumulation. of material at the excitationaperture is prevented.

. Other features and advantages of the present invention will beapparent to persons skilled in the art from the following detaileddescription of a preferred embodiment accompanied by the attacheddrawing wherein identical reference numerals will refer to like elementsin thevarious views.

THE DRAWING FIG. 1 is a side elevational view of a microwave applicatorconstructed according to the present invention;

FIGS. 2 and 3 are respectively left and right end views of theapplicator of FIG. 1,

FIGS. 4 and 5 are perspective views of the applicator of FIG. 1 showingit closed and opened respectively;

FIGS. 6 and 7 are views of the applicator diagrammatically illustratingthe orientation andintensity of the electric field vector within thecavity; and

FIG. 8 is a perspective view of liners that may be used in the cavity ofthe present invention.

DETAILED DESCRIPTION As used herein, the word cavity" refers to theconductive wall members of the applicator as well as the volume definedthereby, and the volume alone is sometimes referred to as the heatingchamber. The entire metallic structure surrounding the heating chamberis sometimes called the cavity housing. The applicator includes thecavity as well as the other elements attached to the housing and themicrowave feed and excitation structure.

In the drawing, the applicator is generally designated by referencenumeral 10 and, in the illustrated embodiment, the applicator takes theform of a right circular cylinder divided into semi-cylindrical sections11 and 12. Each of the sections 11 and 12 is integrally formed of aconductive metal such as aluminum, and each preferably has an interiorcoating of material of still lower resistivity, such as silver. if asilver plating is used, a thickness of about 0.2 mils is sufficient toexceed the skin depth of the currents in the cavity walls at 2.45 GHzand thereby to provide the necessary shielding from the bulk portion ofthe metal comprising the housing of the resonator. The use of metalswith an even greater conductivity will result in a correspondingincrease in efiiciency.

The housing section 1 l is provided with a pair of apertured ears 13,14;and the other housing section 12 is provided with a corresponding set ofapertured ears 15,16. A hinge pin 17 is fitted through the apertures inthe cars 13 and 15, and a similar pin 18 is fitted through the aperturesin the ears l4 and 16 so that the sections 11,12 may be rotated apartabout these hinge pins as shown in FIG. 5 to pemiit access to theinterior of the cavity. Although in the illustrated embodiment thesections are separated along a common plane, persons skilled in the artwill appreciate from the entire disclosure that these sections need beseparated only along two radially extending planes passing through theaxis, or simply radial" planes.

As seen best in FIG. 5, the section 11 includes asidewall 11a having asemi-cylindrical shape and transverse end plate sections 1 lb and 1 1c.Similarly, the section 12 includes a semicylindrical sidewall 12a andtransverse end plates 12b and 126.

About the center of each of the semi-cylindrical transverse end sectionsor plates 11b, 11c, 12b, and 121:, there if formed an aperture,designated respectively 20, 21, 22 and 23. About each of the apertures23 and extending outwardly from its associated transverse end section,there are attached semicylindrical neck members designated respectivelyby reference numerals 24-27. Thus, when the housing sections of theresonant cavity are closed as illustrated in FIG. 4, the side neckmembers 25 and 27 form an entrance 28 for a filament, designated 29 andpassing along the axis of the cavity. Similarly, the side neck members24 and 26 form an exit aperture generally designated by referencenumeral 30 in FIGS. 1 and 2.

Turning now to FIG. 4, portions of the adjacent edges of the sections 11and 12 are cut away to form a widened slot generally designated 32 whichextends in a radial plane passing through the axis of the cavity (alongwhich the string 29 extends) and communicates the axis of the cavitywith the exterior thereof. The slot 32 also extends through the neckmembers 24, 25, 26 and 27; and it permits threading of the filament 29into its operative position without having to open the two sections ofthe cavity housing.

First and second spring clips 33 (see FIG. 5) and 34 of similar shapeare placed about the respective pairs of neck members 25, 27 and 24, 26to resiliently bias or hold the halfsections 11 and 12 in a closedposition.

As best seen in FIGS. 1, 2 and 5, a pair of opposing flanges 35, 36 arewelded respectively to the outer surfaces of the transverse end sections11b and 12b and extend radially outwardly from the respective neckmembers 24 and 26. The flange 36 defines an internally threaded aperture37 for receiving an externally threaded mating stud or bolt 38. Byscrewing the bolt 38 through the aperture 37 of flange 36, it engagesthe underside of flange to force the half-sections ll, 12 apart againstthe action of the spring clamps 33, 34 to tune the resonant cavity tothe operating frequency of the microwave source, as will be more fullyexplained within.

Turning now to FIGS. 3,4 and 5, similar flanges 40, 41 are weldedrespectively to the exterior surfaces of the transverse end sections 11cand 12c; and these flanges also extend radially outwardly from the neckmembers 25 and 27 respectively. A limit switch 43 is mounted on thetransverse end section 110, and it includes a plunger 44 which extendsthrough an aperture 45 in the flange 40. The plunger 44 engages theinner surface of the opposing flange 41 when the housing sections 11, 12are closed, as seen by the dashed lines at 44a in FIGS. 5. When thehousing sections 11, 12 are opened beyond a predetermined limit theplunger 44 disengages the flange 41 and opens the switch 43 to shut offthe source of microwave energy, which preferably includes a magnetronoscillator tube. The operation of switch 43 does permit sufiicientmovement between the housing sections to tune the resonant frequency ofthe cavity over the desired range before shutting off the microwavesource.

Turning now to FIGS. 1 and 2, a probe generally designated by referencenumeral 47 is attached to the transverse end section 11b, and itincludes a center conductor or lead 48 (FIG. 5) which extends into theinterior of the cavity for coupling a small amount of microwave energyfrom the cavity to monitor the power within the cavity. As will be madeclear from subsequent discussion, the probe 47 is located toward theperiphery of the transverse end plate 11b and at a location of minimumelectric field intensity so as to couple a relatively small amount ofpower from the cavity for monitoring purposes. The probe is useful, incombination with an oscilloscope (not shown) to determine the resonantfrequency of the cavity by adjusting the tuning screw 38 to vary theopening between the half suctions 11, 12 in tuning the frequency of thecavity to the frequency of the microwave source. That is, by observingthe power level on an oscilloscope coupled to the cavity by the probe47, one knows that the resonant frequency of the cavity is tuned to thatof the source when the power level is at a maximum.

Turning now to the structure which couples the microwave energy toexcite the cavity, the partially illustrated block 50 is a diagrammaticrepresentation of the housing of a source of microwave energy whichpreferably includes a magnetron oscillator tube; and the outputmicrowave energy is coupled by means of a flanged rectangular waveguide51 to a similarly flanged end termination section 52 which, as seen inFIG. 4, includes a peripheral flange 53, a side wall 54 of rectangularcross section and a transverse end plate 55. An aperture providing acoupling iris 57 is formed in the transverse end plate 55 in line with acorresponding aperture 58 formed in the semi-cylindrical side wall 1 1aof the housing section 11.

As seen in FIG. 1, a second waveguide 59 is connected to a broadwall ofthe feed guide 51 and couples the feed guide to a terminating load whichis schematically designated in the drawing by the E-plane Tee" formed bythe guides 51 and 59 for a stabilizing network, block 59a. The dimensionof the side wall of the coupling wave guide 59 may be one-half thedimension of the side wall of the feed wave guide 51. The stabilizingnetwork may be constructed. according to the manner disclosed inMicrowave Electronics by .l. C. Slater (Van Nostrand Co. 1950) at aboutp. 200 or according to the disclosure of the article of H. F. Huangentitled Microwave Apparatus For Rapid Heating of Threadlines" andpublished in the Fourth Symposium of International Microwave PowerInstitute (I969). The function of the stabilizing network is to insurethe generation of power by the magnetron at the resonant frequency ofthe cavity. It has been found that, because of the impedance of thecavity is so highly frequency dependent, power can be generated atfrequencies other than the resonant frequency of the cavity and,therefore, never accepted by it.

Turning now to FIG. 8, there are shown first and second semi-cylindricalliners 60 and 61 made of a material having a low dielectric loss factorsuch as rigid polyethylene. Since there is some wobble in transportingthe filament through the cavity at fairly high speeds (of the order of50-100 feet per section) in those applications wherein the filament isim' pregnated with a liquid such as latex and it is desired to dry thefilament, there will be some splashing of the impregnated material on tothe side walls of the cavity where the material will collect. For theseapplications and for other applications in which the interior of thecavity is likely to be coated during operation, the plastic liners 60and 61 have been found to be advantageous in that they can be removedfrom the cavity and replaced by clean liners to minimize the down timeof the equipment.

Turning back to FIGS. 1 and 2, air is passed by a fan (not shown) overthe magnetron for cooling the tube, and this heated air, the flow ofwhich is represented by the arrows 63, is passed from the housing 50 ofthe microwave source through the feed waveguide 51, the terminationsection 52, the aperture 57, and into the interior of the heatingchamber from which it is forced either through the slot 32, or theentrance or exit apertures 28, 30. As mentioned in the aboveidentifiedcopending application, Ser. No. 852,374, the combination of forced airinto the cavity and the microwave heating achieves much better resultsthan the heating alone, not only to carry moisture away, but to causethermal equilibrium within the cavity. That is, if the cavity wallschange in temperature, the resulting expansion will cause a shift in theresonant frequency of the cavity. It has been found that more effectivedrying results are achieved by preheating the air, for example asdisclosed herein, by passing it over the operating magnetron oscillatortube. In the particular arrangement illustrated, namely wherein the airpasses through the coupling aperture between the feed waveguide and thecavity sidewall, the forced air also serves to purge the resonator andfeed guide of vapors and thus inhibit the collection of material aboutthe coupling aperture and the interior of the feed waveguide 51.

Before discussing the nature of the electromagnetic field within thecavity, the features of the above-described structure will besummarized. The cavity is fabricated in two separate sections which arehinged about an axial edge opposite the threading slot 32 for opening.This arrangement allows the cavity to be conveniently opened forcleaning or removing of liners and, secondly, it permits the tuning ofthe resonant frequency of the cavity. Tuning is accomplished by means ofthe thumb screw 38 threadably received in the flange 36 and actingagainst the flange 35. The sections of the cavity housing are biased ina closed position by means of the clamping springs 33 and 34.

An electrical switch is provided to sense when the cavity is opened toprovide an interlock so that microwave power cannot be generated withthe cavity opened. However, there is no hazard if the interlock is notprovided because if either the filament 29 is not present or the cavityis opened, the mismatch between the cavity and the source is so greatthat very little energy is coupled to the cavity. Under normal operatingcircumstances with about 1,000 watts of microwave power generated by themagnetron and the interlock system defeated, radiation of microwaveenergy at the plane of the slot with the cavity open was greater than dbbelow the acceptable density of 10 mw/cm.

By routing the air used in cooling the magnetron through the feedwaveguide and exhausting it into the applicator at the couplingposition, the incoming air is preheated and serves to purge theresonator vapors, to inhibit buildup about the input coupling iris andfeed waveguide, and create conditions of thermal equilibrium within thecavity.

Turning now to the electromagnetic field within the cavity, if oneselects the dimensions of a conducting right cylindrical resonator withregard to the wavelength of the excitation frequency, the resonantfrequency of a specific electric field pattern within the resonator canbe made to match the output frequency of the magnetron that excites thecavity. The mode used in the hinged applicator of the present inventionis the lowest order mode, referred to as the TM mode. As schematicallyillustrated in FIG. 7, the electric field vector (designated by thevertical arrows 65) extends parallel to the axis of the cylindrical sidewall; and this is referred to as the z direction. The magnitude orintensity of the electric field varies with the radius, r, or distancefrom the axis according to the following equations:

2 0( c wherein E the electric field intensity at r= 0;

k 2.405/a (where a is the radius of the cavity);

r= the radial position from the axis; and

J the Bessel function of first kind, order 0.

Ez is independent of the longitudinal position along the axis,

2, and the angular position, D. If the filament is run parallel to the zdirection at the position r O (or a web fed transversely through theaxis), the material will be located at the position of maximum electricfield intensity and, hence, it will experience the maximum heating ratewhich is proportional to the square of the electric field intensity.Moreover, in the unperturbed case, this intensity is substantiallyuniform in the z direction, and the actual heating rate as a function ofz will not vary appreciably due to large changes in the electric fieldintensity. That is, the heating rate will be primarily a function of thedielectric loss properties and/or the moisture present as a function ofaxial distance, z.

In this particular mode, the magnetic field lines (representedschematically by the dashed circular lines in FIG. 6) extend about theaxis of the cavity, and they increase in intensity in proceeding fromthe axis to the sidewall, pictorially demonstrated by the closeness ofthe dashed circular lines. It can also be seen diagrammatically fromFIG. 6 that the intensity of the electric field is at a maximum at thecenter of the cavity. In FIG. 6, the electric field vectors arerepresented by the small circles. The currents flow along the innersurface of the cavity as schematically illustrated by the lines 70, andit will be appreciated that the currents are substantially equal for allangular positions about the interior surface of the cavity.

In addition to allowing the slot opening for threading the strand withminimum radiation during operation, the current orientation of this modepermits the applicator to be split completely apart as illustrated inFIG. 5 at the plane of the slot (although as already mentioned, theradial planes of the two slots need not be coextensive).

In a preferred embodiment a JC 300 magnetron tube manufactured by theGeneral Electric Co. is used as the power source. It operates at afrequency of 915 MHz. and a power level of 1.0 Kw. The feed waveguide isof the type known in the art as WR 975. About one-third of the power isconsumed in the stabilizing network load and the rest absorbed in thecavity. The cavity is about 10in. in diameter and 8 in. in axial length.The inlet and outlet apertures for the filament are 1.5 in. in diameter.The width of the threading slot is variable from an almost closedposition to about one-fourth in. at the edge. The amount of air mayrange from to cubic feet per minute.

It will be appreciated by persons skilled in the art that the cavitiesof the type disclosed herein may be used in tandem so that the filamentpasses through a first cavity and then directly through a second,following cavity in the same manner as illustrated above. In a situationsuch as this, the second cavity could be made somewhat longer to moreevenly divide the power consumption because the filament will be moredry upon entering the second cavity.

Persons skilled in the art will appreciate that the inventive principlemay also be employed in an applicator of the type described above butadapted to receive a web to be dried or a conveyor supporting article tobe dried wherein the web is moved in a plane passing through the axisi.e., in the area of maximum electric field intensity in theneighborhood of the axis. Such a system is disclosed in my copending,co-owned application for Microwave Applicator For Heating ContinuousWeb, Ser. No. 860,657, Filed Sept. 29, I969. Further, although the TMmode of excitation is preferred, other higher order modes may as well beused wherein the maximum field intensity extends along the axis of thecavity.

Having thus described in detail a preferred embodiment of the inventiveprinciple, it will be apparent to persons skilled in the art thatcertain modifications may be made to the structure illustrated and thatelements may be substituted for those disclosed; and it is, therefore,intended that all such modifications and substitutions be covered asthey are embraced within the spirit and scope of the appended claims.

I claim:

1. In a system for applying microwave energy to material, thecombination comprising resonant cavity means including a cylindricalside wall and formed by first and second separatable housing sections,hinge means pivotally securing said first and second sections about ahinge line extending generally parallel to the axis of said side wallwhereby said sections may be moved to a closed position in which thesides of said sections form the general shape of a cylinder or splitapart to gain access to the interior of said cavity, microwave energysource means for exciting said cavity to generate an electric fieldwithin said cavity extending generally parallel to the axis of said sidewall when closed and having a maximum intensity in the neighborhood ofsaid axis, said housing sections defining aperture means fortransporting said material through the axis of said cavity.

2. The system of claim 1 further comprising means for continuouslyforcing air into the interior of said cavity to purge the same ofmoisture vapor.

3. The system of claim 2 wherein said last-named means includes meansreceiving heated air passed over the source of microwave energy andforcing the same through said excitation means and into said cavity tobe exhausted through said aperture means at a rate in the range of80-120 cubic feet per minute.

4. The system of claim 1 wherein said housing sections aresemi-cylindrical in form and are separated along a plane passing throughsaid axis and wherein said cavity further includes transverse end platesat each end of each of said sections, said end plates defining inlet andoutlet apertures for permitting passage of a resonent along the axis ofsaid cavity.

5. The system of claim 1 further comprising means for resiliently urgingsaid housing sections in a closed position, and further comprisingadjusting mechanism including screw thread means secured to one of saidhousing sections and engaging the other for selectively moving saidsections relative to each other about said hinge means to tune saidresonant frequency of said cavity.

6. The system of claim 1 further comprising first and second neckmembers secured respectively to said first and second housing sectionsadjacent said aperture means through which said material passes, springmeans engaging said neck members for urging the same together to biassaid housing sections in closed position, and adjustable means securedto one of said housing sections and engaging the other section to forcesaid sections apart against the action of said spring means to adjustthe angle between said sections in tuning the resonant frequency of saidcavity.

7. The system of claim 1 further comprising electrical interlock meansresponsive to the separation between said housing sections forpreventing the excitation of said cavity by said microwave source whensaid sections are open beyond a predetermined limit.

8. The system of claim 7 further comprising probe means mounted on saidcavity housing and including a probe extending into the cavity adjacentthe cylindrical side wall for monitoring the energy therein in tuningthe resonant frequency of said cavity to the frequency of the microwavesource.

9. The system of claim 1 wherein said excitation means includes arectangular waveguide defining an iris in register with a correspondingaperture in the side wall of said cavity for exciting the same in the TMmode.

10. The system of claim 1 wherein said housing sections are separatedalong one plane extending through said axis and wherein said material isa filament and said aperture means includes an inlet and an outletaperture for passing said filament along said axis.

1 1. The system of claim 10 wherein the separation between said cavitysections from the axis to the side opposite said hinge means permitsthreading of said filament.

12. in a system for applying microwave energy to a filament,

the combination comprising resonate cylindrical cavity means excited bymicrowave source means to generate an electric field having maximumintensity along the axis of said cavity and provided with transverse endplates each defining an axial aperture for permitting passage of saidthread through the axis of said cavity and along the region of maximumfield intensity of said cavity when excited, said cavity being dividedinto first and second separatable housing sections, and tuning means forpredetermining the separation between said halves about a hinge line totune the resonant frequency of said cavity to the frequency of a sourceof microwave energy exciting the same.

13. The system of claim 12 wherein said tuning means further comprisingresilient means urging said housing sections together and rigid meansfor setting the minimum distance at which said sections may close.

14. In a system for applying microwave energy the combination comprisingresonant cavity means including a cylindrical side wall and providedwith aperture means to pass the material being treated through the axisthereof, microwave source means including feed means connected to saidcavity for exciting said cavity to generate an electric field having amaximum intensity along said axis, and means for passing air throughsaid feed means and into said cavity to purge vapors from said cavity atleast partially through said aperture means and to minimize thecollection of material about the connection between said feed means andsaid cavity means.

15. The system of claim 14 wherein said feed means includes hollowwaveguide means exciting said cavity through a coupling aperture andwherein said air is preheated and passed through said waveguide and saidcoupling aperture.

16. The system of claim 14 wherein said cavity is characterized byhaving first and second semi-cylindrical sections separatable about ahinge line along the side wall thereof to facilitate access to theinterior of said cavity.

17. The system of claim 14 wherein said source means includes amagnetron oscillator, and further comprising stabilizing network meansconnected in circuit with said cavity and said source means for causingsaid magnetron to oscillate at the resonant frequency of said cavity andthereby establish the coupling of power to said cavity from said source.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3.673.370 DatedJune 2?.i972

Patent No.

Inventor s) RAY M. JOHNS ON It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 75, "there if formed" should read --there is formed-.Column 3', lines 44 and 45, "Figs. 5" should read --Fig. 5--. Column 5,line 23, "resonator vapors" should read --resonator of va ors--. Column6, Claim 4, line 74, "passage of a resonent should read --passage of afilament--.

Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER JR A'ttesting Officer ROBERT GOTTSCHALK Commissionerof Patents FORM 5 0-1050 (10-69) USCOMM-DC 603764 69 U.S, GOVERNMENTPRINTING OFFICE: I969 O-366-334 UNITED STATES PATENT OFFICE CE 5 THECATEOF 0" ECT-lQ-N' Patent No. 3 ,673 ,370 I Jated June 27. 1972 RAY M..JOHNSON Inventor(s) It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 75, "there if formed" should read -there is Column 3,lines 44 and 45, "Figs. 5" should read --Fig. 5- Column 5, line 23,"resonator vapors" should read --resonator of va ors--. Column 6, Claim4, line 74, "passage of a resonent should read --passage of afilament--.

formed---o Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GO'I'TSCHALK Attesting OfficerCommissionerof Patents USCOMM-DC 60376-P69 FORM PO-1050 (10-69) u.s.covsmmem PRINTING OFFICE: was oasa-aa4

1. In a system for applying microwave energy to material, thecombination comprising resonant cavity means including a cylindricalside wall and formed by first and second separatable housing sections,hinge means pivotally securing said first and second sections about ahinge line extending generally parallel to the axis of said side wallwhereby said sections may be moved to a closed position in which thesides of said sections form the general shape of a cylinder or splitapart to gain access to the interior of said cavity, microwave energysource means for exciting said cavity to generate an electric fieldwithin said cavity extending generally parallel to the axis of said sidewall when closed and having a maximum intensity in the neighborhood ofsaid axis, said housing sections defining aperture means fortransporting said material through the axis of said cavity.
 2. Thesystem of claim 1 further comprising means for continuously forcing airinto the interior of said cavity to purge the same of moisture vapor. 3.The system of claim 2 wherein said last-named means includes meansreceiving heated air passed over the source of microwave energy andforcing the same through said excitation means and into said cavity tobe exhausted through said aperture means at a rate in the range of80-120 cubic feet per minute.
 4. The system of claim 1 wherein saidhousing sections are semi-cylindrical in form and are separated along aplane passing through said axis and wherein said cavity further includestransverse end plates at each end of each of said sections, said endplates defining inlet and outlet apertures for permitting passage of aresonent along the axis of said cavity.
 5. The system of claim 1 furthercomprising means for resiliently urging said housing sections in aclosed position, and further comprising adjusting mechanism includingscrew thread means secured to one of said housing sections and engagingthe other for selectively moving said sections relative to each otherabout said hinge means to tune said resonant frequency of said cavity.6. The system of claim 1 further comprising first and second neckmembers secured respectively to said first and second housing sectionsadjacent said aperture means throuGh which said material passes, springmeans engaging said neck members for urging the same together to biassaid housing sections in closed position, and adjustable means securedto one of said housing sections and engaging the other section to forcesaid sections apart against the action of said spring means to adjustthe angle between said sections in tuning the resonant frequency of saidcavity.
 7. The system of claim 1 further comprising electrical interlockmeans responsive to the separation between said housing sections forpreventing the excitation of said cavity by said microwave source whensaid sections are open beyond a predetermined limit.
 8. The system ofclaim 7 further comprising probe means mounted on said cavity housingand including a probe extending into the cavity adjacent the cylindricalside wall for monitoring the energy therein in tuning the resonantfrequency of said cavity to the frequency of the microwave source. 9.The system of claim 1 wherein said excitation means includes arectangular waveguide defining an iris in register with a correspondingaperture in the side wall of said cavity for exciting the same in theTM010 mode.
 10. The system of claim 1 wherein said housing sections areseparated along one plane extending through said axis and wherein saidmaterial is a filament and said aperture means includes an inlet and anoutlet aperture for passing said filament along said axis.
 11. Thesystem of claim 10 wherein the separation between said cavity sectionsfrom the axis to the side opposite said hinge means permits threading ofsaid filament.
 12. In a system for applying microwave energy to afilament, the combination comprising resonate cylindrical cavity meansexcited by microwave source means to generate an electric field havingmaximum intensity along the axis of said cavity and provided withtransverse end plates each defining an axial aperture for permittingpassage of said thread through the axis of said cavity and along theregion of maximum field intensity of said cavity when excited, saidcavity being divided into first and second separatable housing sections,and tuning means for predetermining the separation between said halvesabout a hinge line to tune the resonant frequency of said cavity to thefrequency of a source of microwave energy exciting the same.
 13. Thesystem of claim 12 wherein said tuning means further comprisingresilient means urging said housing sections together and rigid meansfor setting the minimum distance at which said sections may close. 14.In a system for applying microwave energy the combination comprisingresonant cavity means including a cylindrical side wall and providedwith aperture means to pass the material being treated through the axisthereof, microwave source means including feed means connected to saidcavity for exciting said cavity to generate an electric field having amaximum intensity along said axis, and means for passing air throughsaid feed means and into said cavity to purge vapors from said cavity atleast partially through said aperture means and to minimize thecollection of material about the connection between said feed means andsaid cavity means.
 15. The system of claim 14 wherein said feed meansincludes hollow waveguide means exciting said cavity through a couplingaperture and wherein said air is preheated and passed through saidwaveguide and said coupling aperture.
 16. The system of claim 14 whereinsaid cavity is characterized by having first and second semi-cylindricalsections separatable about a hinge line along the side wall thereof tofacilitate access to the interior of said cavity.
 17. The system ofclaim 14 wherein said source means includes a magnetron oscillator, andfurther comprising stabilizing network means connected in circuit withsaid cavity and said source means for causing said magnetron tooscillate at the resonant frequency of said cavity and thereby establishthe coupling of power to said cavity from saiD source.